In drum mix plants where an asphalt-aggregate product is being produced, virgin aggregate or a combination of virgin aggregate and used asphalt-aggregate material are introduced to a rotating drum for heating and mixing. While the aggregate material is being mixed and heated in the drum, liquid asphalt is introduced into the drum to coat the aggregate material. In order to achieve proper coating of the aggregate material, sufficient liquid asphalt must be provided. However, in order to avoid waste, and to meet strict specifications for the composition of the mixed asphalt-aggregate material, liquid asphalt must be provided at a particular rate, depending on the grade composition of the aggregate. Proper coating of the aggregate material thus requires a carefully measured amount of liquid asphalt per unit volume of the selected aggregate material.
Methods for determining the amount of aggregate material being processed through the drum are known in the art. Given the amount of aggregate material, the prior art devices have measured out the necessary amount of liquid asphalt using volumetric displacement meters including gears which are driven by the flow of liquid asphalt at a rate proportional to the flow. Volumetric measurement, however, has inherent disadvantages. For example, volumetric measurement of liquid asphalt is temperature dependent. Since asphalt is a semi-solid material at normal ambient temperatures, it must be heated to maintain it in a liquid, flowable state. However, the hotter the liquid becomes, the more volume it occupies. Volumetric meters are used to deliver a constant volume, and therefore deliver less asphalt by weight when the material is hotter than would have been delivered if the material had been cooler, for a particular volume flow rate setting. Since it is difficult to keep the liquid asphalt at a constant temperature throughout the delivery system, temperature fluctuations lead to inaccuracies and fluctuations in the amount of liquid asphalt being combined with aggregate material. Therefore it is more difficult to comply with strict governmental composition standards. To overcome this problem, some prior art volumetric meters have included expensive temperature compensation circuitry and apparatus.
Also, the meter for measuring out the liquid asphalt is expensive and must be calibrated, and the liquid asphalt must be finely filtered to prevent damage to the meter. As a result, the filter may become blocked by relatively small debris. The accuracy of such meters, which operate by counting revolutions of the gears caused by the flow, is compromised by the fact that pockets of air trapped in the line are not distinguished from the liquid asphalt. Furthermore, conventional meters are generally machined to fine tolerances, and, consequently, are prone to clogging, damage and breakdown. It is therefore often necessary to maintain a second meter to replace a malfunctioning meter lest the entire drum mix plant be shut down during repair of the meter.
With volumetric displacement meters, it is usually necessary to perform an initial calibration operation before the meter may be used. This operation often involves pumping many gallons of liquid asphalt through the meter to thereby correlate the number of turns of the gears of the meter to the amount of liquid asphalt flowing through the meter. This is generally a costly and a time consuming operation.